It is known to provide data communication networks whereby a number of devices which are each capable of sending and receiving data messages can send and receive data messages from and to each other. One commonly used type of data network is a local area network (LAN) in which the data transmitting and receiving devices are all connected to a common transmission medium such as a twisted-wire pair or a coaxial cable.
Use of a common transmission medium in the data network makes necessary some provision for resolving or preventing conflicting attempts to access the transmission medium. One well known technique for preventing conflicts entails providing a master station which sequentially polls each of the other stations, with the other stations being permitting to access the transmission medium only in response to a polling message directed to the particular device which desires to access the medium. Conventional polled networks can provide satisfactory performance when each of the devices requires frequent access to the transmission medium, but if the message traffic is infrequent or "bursty", a polled network does not use the transmission medium efficiently. Also, if the number of devices connected to the transmission medium is large, there may be unacceptably long delays in permitting access to the transmission medium.
It is also known to provide so-called random access control techniques, which are generally more suitable for "bursty" traffic. For example, one known random access technique is referred to as "carrier sense multiple access" (CSMA). According to this technique, when a device has data to transmit, the device "listens" to the transmission medium to determine whether the medium is in use, and if not, proceeds to transmit data. If there is a collision, i.e., if two devices transmit respective data packets that overlap in time, then the terminals retransmit their respective data packets in a manner designed to avoid future collisions, such as by delaying a random period before retransmitting.
A disadvantage of the CSMA technique is that the likelihood of collisions increases as the number of devices attached to the transmission medium and the frequency of traffic increases. This may result in unacceptable delays. In addition, this technique is "non-deterministic" in the sense that it can not be known with certainty that a device will be able to gain access to the transmission medium within any given finite period of time. This may not be an acceptable characteristic in the case of time-sensitive data.
There are particularly difficult challenges in managing a local area network which is used to interconnect video surveillance cameras, camera control consoles, and other devices constituting a closed circuit television surveillance system. Such a LAN may include a relatively large number of devices with only infrequent needs for network access and also a smaller number of devices with frequent need for access. In addition, there may be substantial differences in size among the data packets to be transmitted. With access control techniques that have heretofore been applied to LAN's used in connection with video surveillance systems, it has not been possible to achieve satisfactory performance without severely limiting the number of devices (cameras and controllers) connected to the LAN.
It can be contemplated to "partition" the video surveillance system, by providing separate LANs for respective portions of the video surveillance system, but only at the cost of preventing certain control consoles from controlling video cameras not attached to the same LAN as the control consoles. Thus, partitioning the system is inconsistent with the desirable characteristic of permitting every console to control every camera.